1. Field of the Invention
This invention relates to a process for the production of engine base oils with improved seal compatibility and to engine oils containing carboxylic acid esters and ethers as base oils.
2. Statement of Related Art
Engine oils contain base oils to ensure satisfactory lubrication at high temperatures, effective sealing between piston and cylinder and smooth starting of the engine at low outside temperatures. Known base oils based on mineral oil and also synthetic components, such as poly-.alpha.-olefins and esters, can guarantee satisfactory lubrication. In addition, engine oils or rather their base oils should show neutral behavior towards engine seals in order to prevent unwanted leakage of the engine oil into the engine compartment, for example through shrinkage of the seals. For this reason, polyolefins for example, which unfortunately shrink seals, are combined with esters which are known to lead to swelling in contact with elastomers. With environmental considerations in mind, however, efforts are being made to provide engine oils which show better biodegradability. For this reason, it would be desirable to replace non-readily biodegradable base oils, such as poly-.alpha.-olefins or mineral oils, in engine oils. However, the readily biodegradable esters lead to swelling of the seals which places an unnecessary burden on the seals. Accordingly, there is a need to provide engine base oils which show both better biodegradability than mineral oils and polyolefins and also improved seal compatibility.
It has now surprisingly been found that the requirements stated above are satisfied by engine base oils based on carboxylic acid esters to which ethers have been added.
DE-A-30 38 996 describes thermally stable semisynthetic lubricants of mineral oils and polyol ethers which are obtained by condensation of alcohols, such as pentaerythritol, trimethylol alkanes and/or neopentyl glycol, with alkyl halides.
EP-A-286 141 describes lubricants based on mineral and/or synthetic oil which additionally contain at least one compound bearing at least one quaternary carbon atom and at least one ester and/or ether bond in the molecule. Lubricants such as these are said to have improved load-bearing, lubricating and corrosion-inhibiting properties. Ester and ether compounds mentioned as suitable are those of monopentaerythritol, dipentaerythritol and adamantane derivatives. However, highly branched ether compounds such as these are not readily biodegradable. In addition, there is nothing in this document to indicate that mixtures of the type in question also have improved seal compatibility.